1. Field of the Invention
The present invention relates to stage units for positioning an object, and in particular, relates to a stage unit suitable to position a substrate or an original in a manufacturing apparatus, e.g., an exposure apparatus, for manufacturing a device, such as a semiconductor device or a liquid crystal device.
2. Description of the Related Art
As for stage units, used in exposure apparatuses, requiring high-accuracy positioning, a stage unit including a planar motor has attracted attention. Regarding wafer stages, attention has focused on a twin stage structure in which two movable stages are provided. An exposure operation for exposing a wafer to form a pattern on the wafer is performed simultaneously with a measurement operation for measuring the position of a previously formed pattern, so that an increase in throughput can be expected. Japanese Patent Laid-Open No. 2005-253179 discloses a stage unit including a planar motor. One of circuits for driving a linear motor including many coils is an excitation-switching driving circuit. Japanese Patent Laid-Open No. 8-111998 discloses such a driving circuit.
FIGS. 10A to 10C illustrate a related-art twin stage unit including a planar motor. FIG. 10A illustrates arrays of coils disposed in the x direction within a stator. FIG. 10B illustrates arrays of coils disposed in the y direction. FIG. 10C is a side elevational view of the entire stage unit.
Referring to FIG. 10B, the stage unit includes a first coil array 1 in which first coils having a length in the x direction (i.e., extending in a first direction) are arranged in the y direction and a second coil array 2 in which second coils extending in the x direction are arranged in the y direction. The two coil arrays are disposed adjacent to each other in the x direction.
In addition, a third coil array 3 in which third coils having a length in the y direction (i.e., extending in a second direction) are arranged in the x direction is placed within the stator 4 such that the third coil array 3 covers the first coil array 1 and the second coil array 2 as shown in FIG. 10C. To describe an operation for switching between the coils, reference symbols, e.g., LxA1, RyA5, and other similar symbols are assigned to the respective coils as shown in the figures.
Above the stator 4, a first stage 5 and a second stage 6 are arranged. A first movable member 7 and a second movable member 8, each including permanent magnets, are attached to vertically lower portions of the stages 5 and 6, respectively.
The principle that the movable members including the permanent magnets are allowed to generate a horizontal force (drag) and a vertical force (lift) will now be described in brief with reference to FIGS. 15A and 15B.
The first movable member 7 includes a plurality of magnets. The magnets are magnetized in the directions indicated by dashed arrows and are arranged in a so-called Halbach array. Coils are disposed out of phase with the magnets. In the positional relationship between the magnets and the coils shown in FIGS. 15A and 15B, when current flows through the coils, forces indicated by solid arrows are produced due to the Lorentz force. As for the directions of the forces, when current is supplied to the coils in the vertical magnetic flux density, the coils produce a horizontal drag force. Whereas, when current is supplied to the coils in the horizontal magnetic flux density, the coils produce a vertical force. This is the force that allows the movable member 7 to move away from (i.e., lift off) the stator 4. Referring to FIGS. 15A and 15B, the force allowing the movable member 7 to move away from the stator 4 can be produced by either upper coils 9 or lower coils 10. The force produced by supplying current to the upper coils 9 close to the permanent magnets in the movable member 7 is larger than that obtained by supplying current to the lower coils 10. Accordingly, a current supplied to the upper coils 9 can be lower than that supplied to the lower coils 10. Supplying currents of two types, A phase and B phase, to the coils such that the currents are 90 degrees out of phase can produce constant forces.
As described above, the Lorentz force is produced by the permanent magnets and the currents flowing through the coils, so that the movable stage is driven in six axial directions under its own weight and is then positioned. Assuming that the first stage 5 and the second stage 6 are located as shown in FIGS. 10A to 10C, when the currents are supplied to the coils, filled with black in FIGS. 10A to 10C, facing the first movable member 7 and the second movable member 8, the Lorentz force is produced. In this example, four coils are selected for each stage in each of the x and y directions. Since the number of coils supplied with current varies depending on the size of the movable member, a predetermined number of coils best suited to drive the movable member may be selected.
Referring to FIG. 10C, a space, including the first coil array 1, surrounded by a dashed line corresponds to a measurement station 12 and another space, including the second coil array 2, surrounded by a dashed line corresponds to an exposure station 11. In an upper portion of the exposure station 11, a projection optical system 14, e.g., a projection lens, is provided to exposure a substrate (wafer) 13 mounted on the first stage 5 to form a pattern. In an upper portion of the measurement station 12, a measurement optical system 15 is provided. The measurement optical system 15 includes at least one of a measuring device that measures the position of the surface of the substrate in the direction parallel to the optical axis of the projection optical system 14 and a measuring device that measures the position of a mark formed in the substrate.
A coil switching sequence in the movement of a stage will now be described. FIGS. 11A and 11B illustrate a case where the first stage 5 moves in the −x direction by a distance corresponding to the width of one coil and the second stage 6 moves in the +x direction by the same distance. Each arrow in FIGS. 11A and 11B indicates the direction in which the stage moves.
When the first stage 5 moves in the −x direction, a coil RxB5, indicated by hatching, does not face the first stage 5. The excitation of the coil RxB5 is therefore switched off and that of a coil RxB3 having the same phase relation as that of the coil RxB5 is newly switched on, thus performing excitation switching. Similarly, when the second stage 6 moves in the +x direction, a coil LxA3, indicated by hatching, does not face the second stage 6. The excitation of the coil LxA3 is therefore switched off and that of a coil LxA5 having the same phase relation as that of the coil LxA3 is switched on, thus performing excitation switching. The above-described switching of the excitation of the coils having the same phase relation through a switch in accordance with the position of the stage can continuously produce a drag force.
FIG. 12 illustrates a switching circuit for the first to third coil arrays. A stage controller (not illustrated) gives a current instruction to each current amplifier 16. A first switch group 17 for switching between coils in the second coil array 2, a second switch group 18 for switching between coils in the first coil array 1, and third switch groups 19 for switching between coils in the third coil array 3 are arranged.
FIG. 12 illustrates appropriate switching in the first to third switch groups so that current can be supplied to predetermined coils when the stages are located as shown in FIGS. 11A and 11B. The coils having the same phase relation are grouped, switching is performed between the coils in each group, and the selected coil is excited using a single current amplifier, so that the number of current amplifiers is reduced. To drive the two stages in the exposure station 11 and the measurement station 12, respectively, eight current amplifiers 16 in each station, namely, 16 current amplifiers in total have to be excited.
FIGS. 13A and 13B illustrate a coil switching sequence during a swapping operation in which the two stages are interchanged such that the first stage 5 is moved from the exposure station 11 to the measurement station 12 and the second stage 6 is moved from the measurement station 12 to the exposure station 11.
In the vicinity of the middle of the stator 4, currents supplied to the two stages have to be controlled independently of each other so that the two stages are interchanged. As shown in FIG. 13A, therefore, in the vicinity of the middle of the stator 4, a fourth coil array 22 is located next to one segment of the third coil array 3 in the exposure station 11. In the fourth coil array 22, each pair of fourth coils, each extending in the y direction, is placed in the y direction and the pairs of fourth coils are disposed in the x direction. In the measurement station 12, the other segment of the third coil array 3 is placed next to the fourth coil array 22.
It is assumed that the swapping operation for interchanging the first stage 5 and the second stage 6 is performed such that the first stage 5 travels in an upper portion in FIG. 13A in the vicinity of the middle of the stator 4 while moving in the −x direction and the second stage 6 travels in a lower portion in the figure in the vicinity thereof while moving in the +x direction. When the stages are located above both of the first coil array 1 and the second coil array 2 as shown in FIG. 13B, eight hatched coils have to also be excited.
FIG. 14 illustrates a coil switching circuit for achieving the swapping operation in which the two stages are interchanged between the exposure station 11 and the measurement station 12. Eight current amplifiers 20, surrounded by dashed lines, for swapping are further needed. Thus, 24 current amplifiers in total achieve the swapping operation for interchanging the stages.
As described above, when the excitation-switching driving circuit with the switch arrangement disclosed in Japanese Patent Laid-Open No. 8-111998 is applied to the twin stage unit including the planar motor disclosed in Japanese Patent Laid-Open No. 2005-253179, many current amplifiers are needed during the swapping operation for interchanging the two stages. Disadvantageously, the necessity of the many current amplifiers results in an increase in cost and further results in an increase in footprint of an electrical component rack.